This invention relates to linear beam tubes and more particularly to the electron guns of such tubes.
The electron gun end of a typical linear beam tube is shown in FIG. 1 of the accompanying drawings.
Referring to FIG. 1 the glass or ceramic envelope of the linear beam tube is represented at 1; the cathode of its electron gun is represented at 2 and the anode of its electron gun is represented at 3. The anode 3 is mounted directly upon the first pole piece 4 of a focussing structure at the entrance of a slow wave structure (not represented but to the right as viewed) of the linear beam tube. Commonly but not necessarily this first pole piece 4 constitutes also an end wall of the first cavity of the slow wave structure. In this case the anode 3, mounted as it is on the first pole piece 4, is held at ground potential and the length of the main cathode voltage insulator (i.e. the length L of the envelope 1 electrically between the cathode 2 and the pole piece 4) is determined by the voltage stand-off requirements external to the tube. Where the tube operates in air the length L requires to be longer than would be the case if the gun end of the tube were to be immersed in a dielectric liquid.
With the configuration of FIG. 1, during high voltage arcs occurring between the cathode 2 and anode 3 these electrodes are prone to damage and for this reason the known configuration shown in FIG. 2 of the accompanying drawing has found some favour by virtue of the protection that may be afforded to the electrodes in the face of such high voltage arcs.
Referring to FIG. 2 in this case the anode electrode 3 is isolated from the pole piece 4 and is mounted upon a metal cylinder 5 which cylinder is in turn supported between two insulating lengths of envelope 1' with 1" which are each of length equal to L.
A flange 6 by which the cylinder 5 is mounted, and which is sandwiched between the two lengths of insulator 1'and 1", forms an electrical connection for the anode 3. Between the electrical connection formed by flange 6 and earth is an external limiting resistor 7. In practice, and as shown, the end of the resistor 7 remote from the flange 6 is grounded by being attached to the first pole piece 4. In some cases grounding is effected not via the first pole piece 4 but via a current sensor.
With the construction of FIG. 2, during normal operation the anode 3 is held close to ground potential since there is negligible anode current drawn. However when an anode to cathode arc occurs a relatively large current flows through the limiting resistor 7 which charges the anode up to cathode potential, thus causing the arc to be extinguished. In this case the anode is at a potential other than ground potential only during such arcs.
Whilst the tube illustrated by FIG. 2 is, as regards cathode to anode arcs, a "protected" tube as opposed to the tube illustrated by FIG. 1 which is an "unprotected" tube, a serious disadvantage arising from the construction of FIG. 2 is the added length of insulating envelope wall, i.e. the two portions 1' and 1", between the cathode mount and the first pole piece 4--effectively double that of the construction illustrated by FIG. 1. Whilst this added length is in itself undesirable there is in consequence also a tendency for the gun to be the more susceptible to vibration. Such vibration can give rise to electrical noise which is a serious limitation in some systems.